Oil resistant ethylene vinyl acetate copolymers are well-known synthetic materials formed by copolymerizing ethylene and at least 40 wt % vinyl acetate. The ethylene vinyl acetate copolymers may contain only copolymerized ethylene units and vinyl acetate units or the copolymers may comprise additional copolymerized monomers, for example esters of unsaturated carboxylic acids, such as methyl acrylate or butyl acrylate. The raw polymers, also known as gums or gum rubbers, may be cured by free radical generators such as peroxides, azides, or by use of high energy radiation. Examples of commercially available ethylene vinyl acetate copolymers include Elvax® resin products from E. I. du Pont de Nemours and Company and Levapren® products from Lanxess Corp.
In view of their low cost compared to other oil resistant elastomers, ethylene vinyl acetate copolymers are widely used in the manufacture of wire and cable jacketing as well as in the production of automotive parts such as hoses and seals.
Resistance to heat aging is a particularly desirable property in rubber parts that are used in under the hood automotive applications, e.g. hoses, gaskets, and seals. Because such parts may be exposed to temperatures in excess of 160° C. for periods of time, including up to several hours on a regular basis, degradation of physical properties through oxidative embrittlement can occur. In ethylene vinyl acetate rubbers, this often results in a reduction in extensibility and an increase in hardness and modulus of the rubber article. Such effects are disclosed for example in published disclosure EP1081188. Methods to enhance hot air or heat aging resistance of ethylene vinyl acetate rubbers have involved attempts to identify more effective antioxidant systems. However, there is still a need to improve the high temperature resistance of these copolymers.
It has now been found that it is possible to produce cured ethylene vinyl acetate copolymer compositions of high hardness, strength, and elasticity that exhibit excellent heat aging resistance by dispersing particles of polyamide in a blend of ethylene vinyl acetate copolymer and a peroxide curable polyacrylate elastomer. The peroxide curable polyacrylate elastomer comprises copolymerized units of alkyl acrylate, and an amine or acid reactive monomer selected from the group consisting of unsaturated carboxylic acids, anhydrides of unsaturated carboxylic acids, and unsaturated epoxides. The amine or acid reactive monomer allows the polyacrylate elastomer to compatibilize the polyamide and the ethylene vinyl acetate copolymer, thereby improving physical properties such as strength and elongation to break. Polyacrylate elastomers comprising only polymerized units of acrylate monomers generally exhibit a poor cure response to peroxide. This is because contiguous polymerized units of acrylate monomers may lead to significant chain scission in the presence of free radicals, so the net increase in crosslink density is low. As defined herein, a peroxide curable acrylate elastomer must either comprise at least 0.5 mol % of an unsaturated pendant group which functions as a peroxide cure site monomer, or at least 50 mol % copolymerized units of ethylene. Copolymerized ethylene monomer units act as spacers between polymerized acrylate monomer units to limit β-scission.
A number of ethylene vinyl acetate copolymer-polyamide blend compositions have been disclosed in the prior art. For example, it is known to add uncured ethylene vinyl acetate copolymers (i.e. gums) to polyamides to form toughened thermoplastic compositions. U.S. Pat. No. 4,174,358 exemplifies the use of uncured ethylene vinyl acetate copolymers at levels up to 20 wt % as toughening additives for polyamides. A compatibilizer such as a maleic anhydride grafted ethylene vinyl acetate copolymer may also be included in the ethylene vinyl acetate copolymer-polyamide blend, as disclosed in J. Polymer Science: Part B: Polymer Physics, Vol. 47, 877-887 (2009). The polyamide component in these compositions comprises the continuous polymer matrix and the uncured ethylene vinyl acetate copolymer is a minor additive. When polyamide comprises the continuous phase in the blend the composition generally cannot be processed at temperatures below the melting temperature of the polyamide, or can be processed only with great difficulty at such temperatures.
It is also known to form thermoplastic elastomer compositions comprising ethylene vinyl acetate copolymer and polyamide. For example, U.S. Pat. No. 5,948,503 discloses compositions comprising an uncured elastic polymer, a polyamide in the form of fine fibers, and a polyolefin having a melting temperature from 80° C. to 250° C. In addition, certain vulcanized compositions are disclosed therein.
Thermoplastic vulcanizates comprising ethylene vinyl acetate copolymer and polyamide, in which the ethylene vinyl acetate copolymer is dynamically crosslinked (i.e., crosslinked under shear mixing to create a dispersion of elastomer particles in a continuous phase of another polymer) are also known. Such compositions are disclosed in EP2098566, and may be improved by the use of a coupling agent such as maleic anhydride grafted ethylene vinyl acetate copolymer as disclosed in U.S. Pat. No. 7,691,943.
U.S. Pat. No. 7,608,216 and U.S. Patent Application Publication 2006/0100368 disclose compositions prepared by admixing an uncured elastomer, for example an ethylene vinyl acetate copolymer, with a thermoplastic polymer or another uncured (gum) elastomer. Techniques such as fractional curing, partial dynamic vulcanization, or the use of high performance reinforcing fillers are disclosed to increase the green strength of the uncured or partially cured compound. The admixed compositions may be subsequently crosslinked with a curing agent for the elastomer component.
A number of acrylate rubber-polyamide blend compositions have been disclosed in the prior art. For example, it is known to add uncured acrylate elastomers (i.e. gums) to polyamides to form toughened thermoplastic compositions. U.S. Pat. No. 4,174,358 discloses the use of various uncured acrylate elastomers or ethylene based thermoplastic resins comprising up to 95 mole percent ethylene, such as ethylene/methyl acrylate/monoethyl maleate/ethylene dimethacrylate tetrapolymers or ionomers of ethylene/methyl acrylate/monoethyl maleate terpolymers, as toughening additives for polyamides. The polyamide component in such compositions comprises the continuous polymer matrix and the uncured acrylate elastomer is a minor additive.
U.S. Pat. No. 5,070,145 discloses thermoplastic blends of polyamides with ethylene copolymers comprising copolymerized units of dicarboxylic acid anhydrides and optionally alkyl (meth)acrylates. U.S. Pat. No. 7,544,757 discloses that blends of ethylene-alkyl acrylate polymers may be blended at levels up to 30% by weight in polyamide to produce toughened polyamide compositions.
Blends of uncured ethylene acrylic elastomers, polyamides and powdered metals are disclosed in Japanese Patent 2001-1191387.
U.S. Pat. No. 3,965,055 discloses vulcanizates prepared from a blend of rubber and 2 wt % to 10 wt % of a crystalline fiber-forming thermoplastic, wherein the thermoplastic is dispersed in the rubber component in particles not greater than 0.5 micron in cross section with a length to diameter ratio greater than 2. The high aspect ratio of the thermoplastic particles enables pressureless curing without void formation.
Japanese Patent Application Publication H10-251452 discloses a dispersion of polyamide particles in a hydrogenated nitrile rubber (HNBR) matrix wherein a compatibilizing polymer that may be an ethylene copolymer or an acrylate elastomer is also present. The compatibilizing polymer is ionically crosslinked by metal oxide during mixing with the HNBR and polyamide which prevents the acrylate elastomer from forming a continuous phase. The HNBR component is then cured with a peroxide or with sulfur.
U.S. Pat. No. 6,133,375 discloses blends of functionalized rubbers with thermoplastics in which the thermoplastic component is dispersed in the rubber phase. Following addition of a curative for the rubber, the composition is crosslinked to produce a vulcanized article. Examples of functionalized rubbers which are disclosed include acrylic rubbers such as nitrile-butadiene rubber, hydrogenated nitrile-butadiene rubber, epichlorohydrin rubber, and rubbers on which reactive groups have been grafted, such as carboxylated nitrile-butadiene rubber. Thermoplastics that are disclosed include polyetherester block copolymers, polyurethanes, polyamides, polyamide ether or ester block copolymers, and mixtures of polyamides and polyolefins. In the latter case, ethylene-alkyl acrylate copolymers comprising grafted or co-polymerized maleic anhydride, glycidyl methacrylate, or (meth)acrylic acid units may be used to compatibilize the polyamide-polyolefin blend.
U.S. Pat. No. 4,694,042 discloses an elastomeric thermoplastic molding material containing a coherent phase of polyamide and crosslinked elastomeric polyacrylate core shell polymers.
U.S. Pat. No. 4,275,180 discloses blends of thermoplastic polymers with acrylate rubbers, the blends being crosslinked or crosslinkable by radiation or peroxide. Fillers may be used in amounts of up to 40% by weight of the composition.
U.S. Patent Application 2006/0004147 discloses blends of elastomers, for example acrylate elastomers, with thermoplastic polymers such as polyamides, in which both polymers are coupled and crosslinked by free radicals, e.g., by electron beam radiation. The compositions may comprise a continuous phase of thermoplastic with dispersed crosslinked elastomer particles, or a continuous crosslinked elastomer phase with dispersed crosslinked particles of what was initially thermoplastic.
U.S. Pat. No. 8,142,316 discloses cured blends of elastomers and thermoplastics for use in power transmission belts. The elastomer may be an ethylene acrylic elastomer, and the thermoplastic may be a polyamide. Free radical curatives are disclosed as curing agents.
It is also known to form dynamically cured thermoplastic compositions having a polyamide matrix continuous phase and a cured acrylate rubber phase that is present in the form of discrete particles. Thermoplastic elastomeric compositions comprising blends of polyamide and ionically crosslinked ethylene acrylic rubber are disclosed in U.S. Pat. No. 4,310,638. U.S. Pat. Nos. 5,591,798 and 5,777,033 disclose thermoplastic elastomer compositions comprising a blend of polyamide resins and covalently-crosslinked acrylate rubber.
Polyacrylate rubber-polyamide blend compositions disclosed in Zeon Chemicals L.P., HyTemp® Technical Manual, Rev. 2009-1, p. 46 (2009) are said to improve impact strength of plastics. They may also be used to produce thermoplastic elastomers.
It has now been found that when a dispersion of polyamide particles is present in a blend comprising ethylene vinyl acetate copolymer and peroxide curable polyacrylate elastomer, the resultant compositions, when cured by a free radical generator, exhibit enhanced resistance to physical property loss during heat aging. In addition, such compositions maintain excellent tensile strength, modulus, hardness, and elastic properties such as compression set and elongation at break that characterize conventional ethylene vinyl acetate compositions lacking polyacrylate elastomer and polyamide.